To provide an endoprosthesis having immediate and long term functional reliability continues to be one of the critical objectives in orthopedic research. The typically critical aspect has been the inability to provide a simple, easily-installed, trouble-free anchorage to the medullary canal and the ability to remove the prosthesis without damaging the bone. Many techniques, structures and operating principles which have been proposed heretofore are conveniently characterized as: (1) a stem impacted into the medullary canal; (2) a stem fixed mechanically in situ by screws, pins, expansion mechanisms or the like; (3) a stem fixed by cement, (4) a porous stem gradually and eventually anchored by bone growth into the stem voids; and (5) stems equipped with one or more resilient anchorage expedients.
Each of these prior proposals is subject to shortcomings and disadvantages. Illustrative embodiments of certain of these proposals are in the following U.S. Pat. Nos., viz; Fischer 3,846,846; Rostoker 3,906,550; Kahn 3,938,198; Rybicki et al 4,011,602; Rosenberg 4,013,071; Bokros 4,038,703; Pifferi 4,051,559; Berner et al 4,124,026; Grundel et al 4,167,047; Grell et al 4,177,524; Koeneman 4,292,697; Koeneman 4,314,381; and Harris 4,406,023.
Each of these methods presents problems that can lead to failure of the arthroplasty. The problems are as follows.
In devices that are impacted into the bone canal or held in place by expansion bolts or bone screws bearing against surrounding bone, the actual surface of contact between the prosthesis and the bone may be very small and areas of stress concentration will frequently occur. Clinical observations report loosening of the implant principally due to bone resorption.
Devices requiring the use of cement for anchoring are subject to problems associated with the toxicity of the cement, necrosis of the adjacent bone, incomplete filling of the desired intramedullary space, and the absence of resiliency. In addition, reports of longterm results of cemented total hip replacements with a minimum follow-up of ten years indicate that the loosing rate on the femoral side ranges between 30 to 50 percent. This technique has the disadvantage of having a service life of five to eight years because of the cement bond becoming loose and requiring a revision (replacement of the prosthesis).
Stem emplacements relying on bony ingrowth for attachment of the stem to the bone canal wall require excellent apposition of the stem. Typically, a gap of one millimeter will exceed the growth capacity of the bone. In addition, if motion prior to fixation should occur, fibrous tissue results rather than new bone growth. In a more recent "cement-less" technique for anchoring the stem of a prosthesis in the intramedullary canal, the stem is secured within the lower portion of the canal by an impact-pressed fit (initial weight-bearing) between the stem and hard bone and by a close proximity fit of the upper portion of the stem to the cancellous bone.
In constructing a stemmed implant utilizing bony ingrowth, it has been found that several requirements should be considered. In particular, the implant must have rigid, immediate fixation so that the bone can properly grow into the porous surface of the implant. If motion occurs prior to fixation, fibrous tissue will grow instead of bone. A second requirement is excellent apposition. Typically, a gap of one or two millimeters will exceed the growth capacity of the bone and will not result in bony ingrowth. Thus, widespread and accurate apposition is absolutely essential.
Prior proposals for resilient anchorings involve various expedients utilizing coil springs having portions of their convolutions in contact with the bone, or strips of resilient material mounted on a supporting prosthesis stem and providing resilient backing for rigid elements in contact with the bone cavity. All have limited contact with the bone and lack uniformly distributed equalized pressure in a controlled predetermined amount to substantially all portions of the bone cavity opposite the body of the prosthesis.